LR8
LR8
Application Note
AN-H40
High Voltage Linear Regulators and
Constant Current Sources Using LR8
by Scott Lynch, Senior Applications Engineer
Introduction
Operation
The LR8 is a high voltage 3-terminal adjustable linear regulator.
Intended for operation directly off rectified AC mains, the LR8
operates at input voltages up to 450 volts, making it compatible
with line voltages up to 240 VAC. It’s output voltage adjustability
assures that it can be used in most any application. The LR8 is
ideally suited for low power off-line DC power supplies and
SMPS start-up circuits.
Except for it’s higher voltage rating, the LR8 operates like any
other 3-terminal adjustable linear regulator.
Asimpleresistivedividersetstheoutputvoltagewhileacapacitor
at the output improves transient response and ensures regulator
stability. When applicable, an input capacitor is required to
provide energy storage for rectified AC.
Keeping in mind that the LR8 requires at least a 10V difference
between input and output for proper operation, the minimum
value for CIN is:
Available in a leaded TO-92 package and a surface mount
SOT-89 and D-PAK package, it is ideal for applications where
space is at a premium.
Relevant specifications are shown in the table below.
t
CIN > ILOAD
V
IN(pk) − VOUT −10V
LR8 Specifications
where ILOAD = Load current
= Time between peaks of input
waveform
Input Voltage Range
Output Voltage Range
Output Voltage Accuracy
Power Dissipation
(VOUT +10V) to 450V
1.2V to (VIN - 10V)
±5%
t
VIN(pk) = Peak input voltage
VOUT = Output voltage
TO-92:
0.74W
TO-243AA (SOT-89): 1.6W
TO-252 (D-PAK):
2.5W
0.5 to 10mA
3%
Note that the LR8 requires a minimum of 0.5mA load current for
proper operation. The current through the resistive divider may
be included as part of the minimum load.
Output Current
Load Regulation
Line Regulation
Supply Rejection
0.01%/V
60dB typ @120Hz
LR8 Block Diagram and Typical Application
Needed for protection should VOUT exceed VIN
2
R
VOUT = 1.2v 1+
+10µA • R
2
R
1
LR8
VIN
VOUT
Pass
Element
Supply
Load
R1
Overtemp &
Overcurrent
1.2V
10µA
ADJ
CIN
COUT
≥ 1µF
R2
8
SUPERTEX [ Supertex, Inc ]
